Bent subs are used when earth drilling with downhole motors. There are used to create a bend which results in the bore hole deviating from vertical. In order to avoid having a plurality of bent subs on site, adjustable bents subs were developed in which the magnitude of the bend was adjustable.
There are presently two alternative ways of constructing an adjustable bent sub. One alternative construction is exemplified by U.S. Pat. No. 4,077,657 which issued to Kurt H. Trzeciak. The Trzeciak adjustable bent sub has two tubular members in end to end relation. The two tubular members meet at a mating end formed with respect to a common mating plane which is at an angle to the axis of the part. Means is provided to lock the tubular members to each other at selected angular rotation of the ends with respect to each other. In other words, the angular adjustment is accomplished through angularly offset mating faces. Another alternative construction is exemplified by U.S. Pat. No. 4,813,497 which issued to Kenneth Hugh Wenzel. The Wenzel patent has one tubular member which is telescopically received in a second tubular member. Each of the tubular members is "bent" having a primary axis and a secondary axis. Angular adjustment is accomplished by relative rotation of the tubular members. The tubular members may be rotated to offset the "bends" so the opposed ends of the tubular members are parallel or rotated to create a cumulative bend.
As the art of horizontal and directional drilling evolved, it was determined that the preferred placement of the adjustable bend housing was as close to the drill bit, as possible. Drilling motors operate in an eccentric motion in response to mud pumped from on surface pumps. The bearing assemblies are used to rotate the drill but in a concentric motion. A universal joint type of drive shaft connects the drilling motor and the bearing assembly and is used to convert the eccentric motion of the drilling motor to a concentric motion of the bearing assembly which rotates the drill bit. U.S. Pat. No. 4,813,497 which issued to Kenneth Hugo Wenzel was the first adjustable bent housing which could be placed over the connecting drive shaft between the drilling motor and the bearing assembly. The distinction between an adjustable bent sub and an adjustable bent housing is the ability of the tool to accommodate a universal joint type of drive shaft through its central bore.
An adjustable bent housing is subjected to friction where it comes into contact with the side wall of the borehole. With each of the above described constructions of adjustable bent housing problems are encountered due to contact with the side wall of the borehole. The Wenzel design has one tubular member with an exterior neck portion on which a locking nut is positioned. This neck can break due to the force of friction when the bend of the adjustable bent housing comes into contact with the side wall of the borehole. The Trzeciak design would require significant modification before it could be used as a housing. Even with those modifications, having the entire adjustment occurring on one common plane requires more rotational clearance and therefore exacerbates the problem of friction against the sidewall of the borehole.
Some friction between the body of the adjustable bent housings and the sidewall of the borehole is unavoidable, therefore means must be found to strengthen the adjustable bent housing.